The present invention relates to: a method for forming a ZnO (zinc oxide) film; a method for forming a ZnO semiconductor layer; a method for fabricating a semiconductor device, such as a light emitting device, a light receiving device, and a thin film transistor (TFT), comprising a ZnO semiconductor layer; and a semiconductor device comprising a ZnO semiconductor layer.
ZnO semiconductors are in direct transitions and have the feature that the bandgap is wide (not less than 3.4 eV). Thus, application of the ZnO semiconductors to semiconductor devices, such as light emitting or receiving devices for emitting or receiving light in blue through ultraviolet ranges and TFTs, is under consideration.
Known methods for forming a ZnO semiconductor layer include MBE, sputtering, and laser ablation. Recently, for the purpose of improving the crystallinity of ZnO semiconductor layers, formation of a ZnO semiconductor layer via a buffer layer is under consideration.
It is disclosed that when a ZnO light emitting layer is formed via a ZnO buffer layer containing impurities such as Al and Mg, the ZnO light emitting layer is formed in good crystallinity (for example, JP-A-2000-244014). It is also disclosed that when a ZnO semiconductor layer is formed via a ZnO buffer layer formed at a temperature lower than that in the formation of the semiconductor layer, the ZnO semiconductor layer is formed in good crystallinity (for example, JP-A-2001-287998).
Nevertheless, in the above-mentioned prior art using a buffer layer containing impurities, a source for the impurities such as Al and Mg needs to be provided separately. This causes the problem of an increase in the fabrication cost. Further, when the buffer layer containing impurities is formed by sputtering, a ZnO film containing impurities is formed also on the wall and the like of the process chamber. Thus, in case that the buffer layer and the ZnO light emitting layer are formed in the same process chamber, during the formation of the ZnO light emitting layer, the impurities such as Al and Mg can be released into the process chamber from the ZnO film having been deposited on the wall and the like, whereby these undesired impurities can be incorporated into the ZnO light emitting layer. Further, the impurities contained in the buffer layer can diffuse into the ZnO light emitting layer, whereby these undesired impurities can be incorporated into the ZnO light emitting layer. As a result of incorporation of these undesired impurities, the light emitting characteristics of the light emitting layer can be degraded. In order to reduce such incorporation of the undesired impurities, a process chamber for buffer layer formation and a process chamber for light emitting layer formation may be provided separately. Nevertheless, this causes the problem of an increase in the apparatus cost and, hence, in the fabrication cost.
On the other hand, in the prior art using a buffer layer formed at a lower temperature, the incorporation of undesired impurities is reduced. Nevertheless, the temperature is different between the buffer layer formation and the semiconductor layer formation. Accordingly, in case that the ZnO buffer layer and the ZnO semiconductor layer are formed in the same process chamber, a time for raising the temperature of the substrate is necessary after the completion of the buffer layer formation and before the beginning of the semiconductor layer formation. This causes the problem of an increase in the necessary process time and, hence, in the fabrication cost. In order to improve the throughput, a process chamber for buffer layer formation and a process chamber for semiconductor layer formation may be provided separately. Nevertheless, this causes similarly the problem of an increase in the apparatus cost and, hence, in the fabrication cost.
Electronic devices, such as a TFT, a light emitting device, a piezoelectric device, are constructed by stacking thin films having different electric conductivities, such as an insulating film, a semiconductor film, and a conductor film. ZnO is used for forming insulating films, semiconductor films, and conductor films. When an electronic device is constructed using ZnO, the electronic device is constructed by stacking a ZnO film and other films composed of substances having electric conductivities different from that of ZnO or alternatively by stacking a plurality of ZnO films having electric conductivities different from each other. When a plurality of such ZnO films having different electric conductivities are formed by sputtering, each ZnO film is formed using one of ZnO targets different from each other in the amount of doped impurities (such as Al and Ga) so that the carrier density may be adjusted in each ZnO film.
Nevertheless, the prior art method for forming thin films having different electric conductivities needs the preparation of a plurality of substances (ZnO and other substances having electric conductivities different from that of ZnO) or alternatively a plurality of ZnO targets different in the impurity concentration. This causes an increase in the material cost or in the number of fabrication processes, and thereby results in the problem of a higher cost of the electronic devices.